This remarkable Chandra image gave scientists their
first look at X-rays from Mars. In the sparse
upper atmosphere of Mars, about 120 (75 miles)
kilometers above its surface, the observed X-rays are
produced by fluorescent
radiation from oxygen atoms.

X-radiation from the Sun impacts oxygen atoms, knock
electrons out of the inner parts of their electron
clouds, and excite the atoms to a higher energy level
in the process. The atoms almost immediately return to
their lower energy state and may emit a fluorescent
X-ray in this process with an energy characteristic of
the atom involved - oxygen in this case. A similar
process involving ultraviolet light produces the
visible light from fluorescent lamps.

The X-ray power detected from the Martian atmosphere is
very small, amounting to only 4 megawatts, comparable
to the X-ray power of about ten thousand medical X-ray
machines. Chandra was scheduled to observe Mars when it
was only 70 million kilometers from Earth, and also
near the point in its orbit when it is closest to the
Sun.

At the time of the Chandra observation, a huge dust
storm developed on Mars that covered about one
hemisphere, later to cover the entire planet. This
hemisphere rotated out of view over the course of the
9-hour observation but no change was observed in the
X-ray intensity, implying that the dust storm did not
affect the upper atmosphere.

The astronomers also found evidence for a faint halo
of X-rays that extends out to 7,000 kilometers above
the surface of Mars. Scientists believe the X-rays are
produced by collisions of ions racing away from the Sun
(the solar
wind) with oxygen and hydrogen atoms in the tenuous
exosphere of Mars.